The conversion of petroleum and other similar higher molecular weight hydrocarbon feedstocks into useful lower molecular weight products such as liquid petroleum gas, gasoline, jet fuel and diesel oil is well known in the art. Commonly employed conversion reactions, for improving the quality of various hydrocarbon feedstocks and/or cracking higher molecular weight-higher boiling materials to lower molecular weight-lower boiling products, include hydroprocessing (mild and severe) and hydrocracking. Mild hydroprocessing is typically conducted at a temperature of from 350.degree. C. to 425.degree. C. and at a pressure of from 3.5 to 10 MPa using a fixed-bed catalyst without regeneration. Severe hydroprocessing is typically carried out at higher pressures--from 7 to 21 MPa--and the fixed bed catalyst has a regeneration cycle. The conditions in hydrocracking are similar to those of hydroprocessing except that the severity of the reaction conditions is increased and the catalyst contact times are longer.
The effluent stream from a conversion reactor will comprise a wide range of molecular weight hydrocarbons which can be processed downstream for recovery of hydrocarbon products useful for various purposes. The product recovery train typically combines a means for separating out light end components (e.g. butanes and lighter) and a fractionator tower for recovering the distillate products (e.g. pentanes and heavier). Prior to product recovery, however, reaction heat is generally recovered for preheating the reactor feed stream wherein the effluent stream is cooled and a heavy phase is condensed. The mixed-phase stream thus formed is directed to a separation drum to effect phase separation. Since the feed preheat (and effluent cooling) is typically conducted in two stages, "hot" and "cold" liquid steams of substantially constant pressure and overlapping composition are produced. These liquid streams are then generally recombined, depressurized and directed to the product recovery train for further separation.
Separation techniques typically utilized for recovery of the remaining light end fraction include either a steam stripping or a debutanizer distillation column. The heavy end components can be fractionated into the hydrocarbon distillate products using a low pressure fractionation column. As is well known in the art, either the fractionation column or light end separation column can be placed first in the product recovery train.
Several drawbacks have been noted for both schemes as operated in the prior art (either light end separator or fractionator first). Where the light end separator comprises a stripper column placed upstream of the fractionator, the vessel must be sized to accept the entire reaction effluent stream. Due to the presence of hydrogen sulfide in the effluent stream, the vessel must be constructed from a corrosion-resistant material. Downstream production quantities of light naphtha (for gasoline) are less than that of a fractionation-first process since a portion of the light naphtha product is lost in the stripper overhead stream. Further, the stripper overhead stream cannot be condensed to produce liquid petroleum gas. Thus, a stripper-first recovery process cannot duplicate the product recovery distribution of the fractionator-first scheme.
Alternatively, the light end separator can comprise a debutanizer placed upstream from the fractionator. This recovery scheme also has some serious drawbacks. Again, the column must be sized to accept the entire effluent stream. Due to the presence of the entire hydrocarbon cut, the debutanizer reboiler must be operated at a high temperature--on the order of 340.degree. C. to 370.degree. C. Therefore, the reboiler must be fired since process heat is unavailable at this relatively high temperature.
In an alternative embodiment, the fractionator column can be placed upstream of a debutanizer to avoid having to reboil the heaviest components. Receiving only the overheads from the fractionator, the debutanizer can be sized smaller. However, the fractionator operates at a lower pressure than the debutanizer, so the debutanizer feed must be cooled and recompressed with a corresponding loss of work and increased capital expenditure. Clearly, it would be highly advantageous, particularly from an energy efficiency and capital expenditure standpoint, to avoid recombining separated streams, reheating cooled streams and recompressing lower pressure steams while maintaining product range flexibility.